1. Field of the Invention
This invention relates to side-chain liquid crystal polymers and to electro-optical devices using them.
2. Description of Related Art
Liquid crystal polymers combine the electro-optic characteristics of low molecular mass (`LMM`) liquid crystals, for example with a molecular mass typically below 500 with the visco-elastic properties of polymers. In a manner analogous to LMM liquid crystals, liquid crystal polymers show nematic (N), cholesteric (Ch) and smectic (S) mesophases, but on cooling to lower temperatures experience a transition to a viscous or glassy state rather than to a solid crystalline state. In contrast with LMM liquid crystals, whereas the LMM liquid crystal - solid crystal transition destroys liquid crystalline order, the transition to a viscous or glass state stores it.
Liquid crystal side chain polymers consist of a polymeric backbone from which project mesogenic side groups. The backbone is normally flexible, to allow the side chains to adopt the order of the liquid crystal state, and may be wholly organic, for example a polyacrylate or polymethacrylate, or may be an organo-silicon chain, for example a polysiloxane chain. As with LMM liquid crystals, a wide variety of structures may be conceived for the mesogenic side groups, for example sequences of suitably-linked cyclic groups such as phenyl, cyclohexyl and heterocyclic rings.
Some typical liquid crystal polymer structures are described, for example, in the following references:
1. Anal. Chem. (1985), 57, 651-658. PA0 2. Polymer (1985), 26, 1801-1806. PA0 3. Polymer Communications, (1983), 24, 364-365. PA0 4. Makromol, Chem (1985), 186, 2639-2647. PA0 5. Polym. Sci. Technol (1985), 28 345-350. PA0 6. Makromol Chem Rapid Commun (1985), 6, 57-63. PA0 7. Makromol Chem Rapid Commun (1984), 3, 357-60. PA0 8. Faraday Discuss. Chem. Soc. (1985), 79 paper 10. PA0 9. Published UK Patent Application 2146787A. PA0 10. Mol. Cryst. Liq. Cryst. (1985) 122, 205-218. PA0 (1) Homopolymers PA0 (2) Copolymers
In electro-optical devices, liquid crystal polymers may be used in a number of ways, and a brief review is presented in ref. 8 above. Liquid crystal polymers may be used simply as additives to LMM monomeric liquid crystal materials, but their principal importance derives from the possibility of long-term durable storage of information encoded in the liquid crytalline state, for example in a device by effecting a transition to a liquid crystal state in an addressed area by local heating.
Optical storage in LMM liquid crystal materials is known. The commercially available smectic liquid crystal material S2, available from BDH Ltd., (UK) is an excellent optical storage medium, and is currently used as a standard for liquid crystal optical storage. S2 and LMM liquid crystal materials generally suffer from a number of disadvantages as far as optical storage is concerned, as follows:
1. stored information is often easily lost by mechanical stress.
2. the liquid crystal transition temperature is often inflexible,
3. the ease with which they may be homeotropically aligned makes writing of fine lines a problem, as the material springs back to its aligned form on cooling rather than giving a scattering line (`line shrinkage`) so that resolution is limited at certain writing speeds,
4. grey scaling is often difficult.
There is clearly room for improvement even in S2, and some steps toward this have been taken, see, for example, Birecki et al, SPIE Proceedings 420 June 1983, p 194 (Ref: 11).
In liquid crystal polymers storage of the information may be accomplished in a number of ways. For example, U.S. Pat. No: 4,293,435 describes a device in which information may be encoded in a cholesteric texture of a liquid crystal polymer and stored by allowing the polymer to cool below its glass transition temperature (Tg). This device suffers from the disadvantages that Tg must be above normal ambient temperature (Ta, about 20.degree. C.) and that the polymer may need to be heated as much as 100.degree. C. above Ta for recording of information.
British Patent Specification No: 2,146,787A describes a device which uses a different effect, in which information is stored in a polymer in a viscous state above Tg and which persists over Ta. In such a device, at a temperature Tf (above Ta) a texture change occurs from an anisotropic but opaque state to a more fluid state which is highly birefringent and thus optically transmitting, and may also be aligned by a suitable electric field in a manner analogous to LMM liquid crystals (see FIG. 1). The degree of optical transmission increases with temperature in this fluid region until a temperature Tb, at which the isotropic phase begins to appear, and thereafter transmission drops until the clearing temperature TC is reached, at which the last of the birefringent structure disappears.
On heating the polymer to the vicinity of Tb, an optically transmitting state may be produced which is "frozen in" and stored when the temperature drops below Tf. In a device using such polymers Tg is less critical and may be below Ta, and less heating of the polymer is necessary.
Such devices may exploit the field alignment properties of N, Ch or S liquid crystal phases for storage purposes, but generally smectic polymers are preferred for better order and higher contrast. The quality of the display and the level of storage may be improved by, for example, the incorporation of pleochroic dyes, by laser addressing and by using birefringence effects, for example with crossed polarisers. Some such methods are described in Refs: 1 and 2 mentioned above.
Some efforts have been made to attempt to prepare mixtures of liquid crystal polymers and LMM liquid crystal materials, for example Ringsdorf et al Makromol Chem Rapid Commun. (1982) 3.745 (Ref: 12) reports on miscibility studies of nematic liquid crystal polymers with LMM liquid crystal materials. Miscibility problems were encountered, and no mention is made of the appearance of a biphasic region at Tb. Such mixtures could not have faster responses than the LMM nematic materials and could only offer advantages over nematic polymers. A similar, but rather inconclusive, study is reported in Ref: 8, discussing nematic non-storage applications of such mixtures.